There are two 9-digit numbers on the back of the Global Entry card. The one on the upper left corner is the Known Traveler Number or PASSID. But on the upper right hand corner there is another 9-digit number, slightly larger than the PASSID. What is the purpose of this second 9-digit number?
It is simply the number if the Global ID card. You can see it in the machine readable area.
This allows to make a Global ID card invalid, for example if you lose it, without making your Known Traveller Number invalid, because it represents your person and is still valid.